We analyzed in detail the electric field variations preceding the first return strokes of 80 cloud‐to‐ground lightning flashes in nine different storms observed at the NASA Kennedy Space Center during the summers of 1976 and 1977. The electric field variations are best characterized as having two sections: preliminary variations and stepped leader. The stepped‐leader electric‐field change begins during a transition period of a few milliseconds duration marked by characteristic bipolar pulses. The durations of stepped leaders lie most frequently in the range 6–20 milliseconds. We infer from our measurements and critical review of the previous literature that there is only one type of stepped leader, not the two types, α and β, often referred to in the literature.

In the course of 71 days in lunar orbit, from 19 February to 3 May 1994, the Clementine spacecraft acquired just under two million digital images of the moon at visible and infrared wavelengths. These data are enabling the global mapping of the rock types of the lunar crust and the first detailed investigation of the geology of the lunar polar regions and the lunar far side. In addition, laser-ranging measurements provided the first view of the global topographic figure of the moon. The topography of many ancient impact basins has been measured, and a global map of the thickness of the lunar crust has been derived from the topography and gravity.

A comprehensive study of a three-stroke lightning flash that struck the 150-m weather tower at Kennedy Space Center (KSC) was recently reported by Uman et al. (1978). Here we supplement that study by presenting three-dimensional locations of the sources of VHF (30-50 MHz) radiation generated by the KSC flash. These locations were obtained by measuring with the KSC lightning detection and ranging (LDAR) system the difference in the time of arrival (DTOA) of radiated pulses at four ground stations and then by calculating the locations from the DTOA data. The DTOA data were obtained via a newly developed computer-implemented multiple time series analysis which uses cross-correlation and pattern recognition techniques. About 48,000 sequential VHF sources were located during the flash. We compared these locations with correlated wide band electric field records to provide a better understanding of the physics of the following discharge phases: preliminary breakdown, stepped leader, return strokes, dart leader, stepped-dart leader, J changes, solitary pulses, and intracloud discharge. Some of the more interesting new results obtained follow: before the stepped leader preceding the first return stroke, there was a primarily vertical discharge in the cloud between a height of 7.2 and 5.1 kin. This preliminary breakdown has uniquely different VHF characteristics from those of the stepped leader which followed and was not accompanied by significant electric field change as was the stepped leader. The stepped leader propagated downward from the bottom of the preliminary breakdown. The only significant VHF radiation during the dart leader preceding the second return stroke came from within the cloud. The third return stroke was initiated by a stepped leader which propagated from about 8 km height, the bottom of the previous J process, down to about 3.3 km where it apparently joined the channel of the previous return stroke and became a dart leader. The VHF source locations during the two J changes were associated with the lowering of about 3 C of negative charge at velocities of about 2 x 105 m/s along primarily vertical paths from a height of about 13.7 to about 7.9 km for the first J change and from 11.2 to 7.3 km for the second J change. During the second J change and before and during the intracloud discharge following the three strokes to ground, discharges which appear on the VHF record as isolated or solitary pulses propagated upward 3-8 km at velocities of the order of 10 ? m/s. The intracloud discharge had a length of more than 10 km and was oriented primarily vertically. The VHF radiation during the intracloud discharge exhibited the three phases reported in the literature for cloud discharges not associated with ground discharges. The highest level of VHF activity during the flash occurred during 4.3 ms just after a 2.4 ms quiet period following the first return stroke. These VHF sources were located primarily at the top of the return stroke channel. earthquake studies [Enochson, 1973], and communication VHF (30-5...

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Five major DFT algorithms were evaluated on seven different computers. The relative performances of these algorithms were related to the architecture of each computer by finding a relationship between the execution time and the instruction counts. The relative performance of these algorithms on other Computers is predicted, based on the knowledge of the computer architecture. On certain implementations, data transfers are more important than floating-point additions and multiplications when comparing DFT algorithms. On the average, data transfers account for a greater percentage of the execution time than floating-point operations. W TNTRODUCTION E have been unable to find in the literature an evaluation of the execution time of the basic DFT algorithms as a function of sequence lengths when implemented on different computer architectures. This information is extremely useful for choosing an algorithm and a computer that will be used primarily to process Fourier transforms. We examined five basic DFT algorithms: the radix-2 [l], the mixed-radix [21, the Winograd [3], the prime factor [4], and the radix-4 [5]. Evaluation of these algorithms has been limited to comparing the algorithms on the same or similar computers [4], [6]-[9]. In this paper these evaluations are expanded to include all five algorithms on seven different computers. The instruction counts and execution times are related for each sequence length and each computer architecture by the percentage of time spent on each instruction and by the correlation coefficients. The percentage of time can be used to determine which instruction type has the greatest effect on the execution time. COMPARISON OF DFT ALGORITHMS Each algorithm was divided into sections and analyzed; the number of times each section was executed was then determined [lo]. From this information, the number of executions of each type of assembly language instruction was calculated [ll]. Each algorithm was run for different sequence lengths on seven computers: